(1) Field of the Invention
The present invention relates to method and apparatus for controlling supercharge pressure for a turbocharger.
(2) Description of the Prior Art
A turbocharger is constructed in such a manner that exhaust turbine is rotated by exhaust gas at high temperature and at high pressure energy so that the pressure within the intake manifold increases above atmospheric pressure as the rotational speed of a compressor in the turbocharger increases. As a result, the supply of a large quantity of intake air flow to the engine becomes possible by the supercharge pressure thus obtained, with the result that high torque, high output and improvement of fuel consumption can be obtained.
Now, in a car engine having a wide range of engine speeds, it is possible to sufficiently secure the supercharge pressure in the middle and high speed operating zones. In the low speed operating zone, however, as it is difficult to obtain a sufficient exhaust pressure, the torque at low speed will tend to be insufficient, without deriving the supercharge pressure. In this case, it is known that the determinant of the supercharge pressure in the lower speed operating zone is the ratio A/R where A is the cross-sectional area of a scroll and R the radius from the center of a scroll. Accordingly, if the cross-sectional area can be made small in the low speed operating zone having a small quantity of the exhaust gas flow, the supercharge pressure can be increased by increasing the number of turbine rotations.
To this end, a turbocharger of the variable capacity type which has capacity charging means provided, with the ratio A/R of the turbine being variable, has already been proposed by the same applicant of this application (see, for instance, Japanese Patent Application Ser. No. 58-162918) in which a sufficient supercharge pressure can be obtained even when the turbocharger of the variable capacity type is operated in the low speed operating zone.
In the supercharge pressure control using the turbocharger described above, the provision is made for an actuator for driving the capacity changing means of the turbocharger, with the supercharge pressure being at work pressure or operation pressure which is produced downstream of a compressor and the supercharge pressure is maintained constant at a preset supercharge pressure, i.e., it is maintained constant by controlling the duty value of an electromagnetic valve which discharges the operating pressure to outside.
In the control characteristic of an electromagnetic valve where the X-axis indicates duty value (see, for instance, FIG. 5), the duty value signifies the opening time of the valve per a predetermined time, when it is 100 percent it indicates that it is fully opened and the cross-sectional area A is made minimum in this case by means of the actuator and the capacity changing means, so as to increase the number of turbine rotations.
On the other hand, when the duty value is zero, it indicates that the electromagnetic valve is fully closed, with the result that the sectional area A is made maximum and the number of turbine rotations is suppressed, thus controlling the supercharge pressure to be maintained constant. In the actual control, in this case, in order to overcome the deviation of control involving different dispersion factors, it is common practice to perform a feedback control in response to an actually detected value. In this example as well, the amount of feedback correction is calculated from the deviation between actual supercharge pressure and a target supercharge pressure and the duty value is corrected by this amount.
In order to achieve agreement between the actual supercharge pressure and the predetermined or target supercharge pressure in a variable capacity type turbocharger, a feedback control is performed. In this case, it is desirable to carry out the feedback control only in a controllable zone where the actual supercharge pressure can be made equal to the predetermined or target supercharge pressure. For instance, when the engine operates at low speed and low load conditions, there will be a limitation in the speed of the exhaust gas injected through a nozzle since the absolute amount in the exhaust gas flow is essentially small, even when the duty value is maintained at 100 percent, with the cross-sectional area of A being minimum so that there remains an uncontrollable zone where the actual supercharge pressure does not reach the target supercharge pressure.
If the feedback control, e.g., any of the proportional, integral, or/and differential controls, is carried out in this uncontrollable zone, the deviation of the actual supercharge pressure from the target supercharge pressure, which never reduces to zero, exists, so that the integral portion, i.e., the amplitude proportional to the integral value is especially dispersed. Accordingly when the engine speed increases from this condition, the actual supercharge pressure largely deviates from the target supercharge pressure due to the excessive correction amount and an overshoot occurs. On the other hand, when the actual supercharge pressure becomes excessive beyond the target supercharge pressure, knocking phenomena easily tend to occur.
In order to avoid the dispersion of the integral portion, the feedback control is often carried out. However, when the operation zone is excessively limited, the start of the feedback control becomes delayed. Since only a small value of the proportional portion having an amplitude proportional to the deviation can be obtained, the start of the supercharge pressure becomes delayed at the time of acceleration and the response tends to be low. On the contrary, when the operation zone is excessively expanded, the feedback control becomes too fast, the integral portion of the correction amount to be fed back becomes large as described above and generation of knocking is often brought about due to the overshoot of the supercharge pressure.